This invention relates to audio noise reduction circuits. In particular, this invention relates to speech operated noise attenuation circuits that attenuate intersyllabic background (noise between syllables) in audio circuits such as those found in the audio output stage of a radio receiver.
Prior art speech operated noise attenuation device (SONAD) attenuate an audio signal in the absence of a voice or speech signals to suppress the apparent level of background noise between syllables. These prior art circuits typically provide a degree of intersyllabic noise attenuation which is related to the prevailing noise level in the signal. When background noise levels vary, the noise suppression provided by these prior art circuits varies as well. In many audio signals the level of background noise varies over a wide range so that the effectiveness of the prior art SONAD in removing intersyllabic noise will vary significantly. A SONAD that reduces background noise while accommodating changes in the background noise level would be an improvement over the prior art.